The invention relates to a powder coating composition comprising a binder selected from carboxyl-group-containing polyesters, carboxyl-group-containing poly(meth)acrylates and mixtures of the said substances, and one or more epoxy compounds as thermal hardeners, and also to a preferred preparation process for one type of the epoxy compounds that are to be used.
Powder coating compositions as referred to at the outset are used in a wide variety of forms. Triglycidyl isocyanurate (TGIC) has been successful as an epoxy hardener in such compositions, especially for external paints, which must have a high weather resistance. Its solid consistency, inter alia, has resulted in TGIC being considered today as the standard hardener for powder coating compositions based on carboxyl-group-containing polyesters as binders (see, e.g. Ullmann""s Encyclopedia of Industrial Chemistry, 5th Ed., Vol A9, p. 559) and on carboxyl-group-containing poly(meth)acrylates (see, e.g., Johnson Wax Speciality Chemicals Product Application Bulletin, Powder Coatings).
There have also been known for some time, however, powder coating compositions stable to outside weathering that are based on a TGIC-free, solid mixture of epoxy resins as hardener (see, e.g., EP-A-0 536 085), where substantial amounts of a liquid, higher-functional epoxy resin, e.g. Trimellitic acid triglycidyl ester, are incorporated into a solid epoxy resin, e.g. diglycidyl terephthalate, without the total mixture of epoxy resins taking on a liquid consistency as a result. In industrial practice, however, virtually the only solid resins available hitherto for such hardener mixtures have been difunctional glycidyl esters. Furthermore, the solid resin makes up the majority of such a mixture, so that a significant disadvantage of such hardener mixtures is that their epoxy functionality is appreciably reduced in comparison with TGIC. In addition, clean glycidylisation of 1,2-dicarboxylic acids is not easy on an industrial scale.
Accordingly there is still a need for new powder coating compositions with properties comparable to those of the above-mentioned powder coating compositions from a surface-coating technology standpoint, that is to say, for powder coating compositions that, especially, have good flow behaviour and high reactivity and with which it is possible to produce coatings having a high crosslinking density and a high level of stability towards weathering and UV. The present invention provides such new powder coating compositions.
The invention relates especially to powder coating compositions that comprise a binder selected from carboxyl-group-containing polyesters, carboxyl-group-containing poly(meth)acrylates and mixtures of the said substances, and one or more epoxy compounds, wherein the epoxy compounds comprise at least one compound of formula (I) that is solid at 25xc2x0 C.: 
wherein
A corresponds to a group of formula (II), (III), (IV) or (VI): 
xe2x80x83in which
B is an x-valent organic radical that is derived from a polyol having x or more hydroxyl groups by the removal of x hydroxyl groups;
E is a (2x)-valent organic radical that is derived from a polyol having (2x) or more hydroxyl groups by the removal of (2x) hydroxyl groups; and
D is a (y+2z)-valent radical that is derived from a polyol having (y+2z) or more hydroxyl groups by the removal of (y+2z) hydroxyl groups;
R1 and R5 are each independently of the other hydrogen, halogen, C1-C4alkyl or C1-C4alkoxy or are together a methylene group; and
R2, R3, R4, R6, R7, R8 and R9 are each independently of the others hydrogen, halogen, C1-C4alkyl or C1-C4alkoxy; and
x is an integer of at least 3;
y is an integer from 1 to (xxe2x88x921) and
z is (xxe2x88x92y).
The powder coating compositions according to the present invention are distinguished, inter alia, by a very good flow behaviour, and yield a cured material that has a high crosslinking density, a high degree of fastness to weathering and a high gloss. Epoxy resins of formula (I) are, in addition, toxicologically less harmful than glycidyl compounds such as are normally used for powder coating compositions.
When one of the radicals R1, R2, R3, R4, R5, R6, R7, R8 and R9 in formula (I) is halogen, it is preferably, for example, chlorine or bromine; when one of those radicals is C1-C4alkyl or C1-C4alkoxy, it is, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl or tert-butyl or an alkoxy group corresponding to one of those alkyl groups.
Preferably, the radicals R1, R2, R3, R4, R5, R6, R7, R8 and R9 are C1-C4alkyl or, especially, hydrogen.
At least some of the compounds of formula (I) are known or are obtainable in known manner or in a manner analogous thereto.
Compounds of formula (I) wherein A corresponds to a group of formula (II) can be obtained, for example, from a polyol of formula B(OH)x wherein x is as defined hereinbefore, by esterifying the x hydroxyl groups of the polyol with cyclohexene-3-carboxylic acid and then epoxidising the carbon double bonds of the resulting polyester compound in customary manner, for example by means of an organic peracid, such as, for example, peracetic acid.
An especially preferred process for the preparation of compounds of formula (I) wherein A corresponds to a group of formula (II) comprises the transesterification of a cyclohexene-3-carboxylic acid ester, especially a cyclohexene-3-carboxylic acid C1-C4alkyl ester, such as methyl 3-cyclohexenecarboxylate, with a polyol of formula B(OH)x, wherein x is as defined hereinbefore, in the presence of LiNH2 as transesterification catalyst, and with continuous removal from the reaction mixture of the alcohol freed from the cyclohexene-3-carboxylic acid ester, the transesterification being followed by the epoxidation of the carbon double bonds of the resulting transesterification product, which is carried out in customary manner, for example by means of an organic peracid, such as, for example, peracetic acid. The use of LiNH2 as catalyst results, inter alia, in especially good yields and a high degree of product purity. The said process can also be used for epoxy compounds of formula (I) wherein A corresponds to a group of formula (II) and x is 1 or 2, and the present invention relates also thereto.
Compounds of formula (I) wherein A corresponds to a group of formula (III) can be prepared, for example, in accordance with British Patent No. 870 696, by reacting a polyol of formula B(OH)x, wherein x is as defined hereinbefore, with an aldehyde of formula (V) 
wherein R1 and R5, as well as R2, R3, R4, R6, R7, R8 and R9, are likewise as defined hereinbefore, in the presence of a suitable catalyst, such as, for example, p-toluenesulfonic acid, and epoxidising the carbon double bonds of the resulting product in customary manner, for example by means of an organic peracid.
Compounds of formula (I) wherein A corresponds to a group of formula (IV) can be obtained, for example, in accordance with SU-A-1 792 956, by trimerising an aldehyde of the above-mentioned formula (V) in the presence of an acid, for example phosphoric acid or nitric acid, and epoxidising the double bonds of the resulting product, again in customary manner.
Compounds of formula (I) wherein A corresponds to a group of formula (VI) are likewise known, for example from Batog, A. E.; Pet""ko, I. P.; Kozlova, L. V.; Pandazi, I. F.; Plast. Massy (1979); (10), p. 9-10, where, for example, a compound of the above-mentioned formula (I) is described in which A corresponds to the group set out below and D is a tetravalent radical derived from pentaerythritol by the removal of 4 hydroxyl groups: 
Preference is given to powder coating compositions according to the invention wherein A corresponds to a group of formula (II), especially where x is from 3 to 6 and, preferably, is 4.
B in formula (II) is preferably a radical that is derived from an aliphatic polyol having from 3 to 20 carbon atoms, from a cycloaliphatic polyol having from 5 to 20 carbon atoms or from a mixed aliphatic-cycloaliphatic polyol having from 7 to 20 carbon atoms.
More especially, the radical B in formula (II) is derived from 1,3-dihydroxy-2,2-di(hydroxymethyl)propane (pentaerythritol).
Preference is given also to powder coating compositions according to the invention wherein A corresponds to a group of formula (III), especially where x is from 3 to 6 and, preferably, is 3.
E in formula (III) and D in formula (VI) are each preferably a radical derived from an aliphatic polyol having from 3 to 20 carbon atoms, preferably 5 or 6 carbon atoms.
The radical B in formula (III) is derived especially preferably from a polyol selected from mannitol, especially D-mannitol, sorbitol, especially D-sorbitol, and dulcitol.
Powder coating compositions wherein A corresponds to a group of formula (IV) also constitute a preferred embodiment of the invention.
Another special embodiment of the powder coating compositions according to the invention is one which comprises at least one further epoxy compound of formula (I) that is solid at 25xc2x0 C. wherein
A corresponds to a group of formula (II) or (III) and
x is 2.
For the radicals R1 to R9, and also for the groups B and E, the same applies in the case of epoxy compounds of formula (I) in which x is 2 as in the case of the other epoxy compounds of formula (I), in so far as the meanings are compatible with the value x=2. Examples of epoxy compounds of formula (I) wherein x is 2 that are suitable in accordance with the invention include, inter alia: 
The preparation of such difunctional epoxy compounds can likewise be carried out in the manner already described above for the corresponding trifunctional and higher-functional compounds.
The epoxy compounds of formula (I) wherein x is at least 3 and the epoxy compounds of formula (I) wherein x is 2 can be present in the powder coating compositions in a widely variable molar ratio, for example in a molar ratio of up to a maximum of 1:2, preferably up to a maximum of 1:1, especially a maximum of 1:0.5.
The powder coating compositions according to the invention may in principle also comprise, in addition to the epoxy compounds of formula (I), certain amounts of one or more other epoxy compounds, e.g. glycidyl esters, such as those described in EP-A-536 085, EP-A-770 605 and EP-A-770 650. The expression xe2x80x9ccertain amountxe2x80x9d is to be understood as meaning that a maximum of 60 percent, preferably a maximum of from 5 to 30 percent, of the total epoxy groups of the powder coating compositions according to the invention is provided by those other epoxy compounds. Especially preferably, however, the powder coating compositions according to the invention are substantially free of such other epoxy compounds, especially glycidyl compounds, such as TGIC, or glycidyl esters, such as diglycidyl terephthalate, or the corresponding glycidyl methacrylates or copolymers thereof. xe2x80x9cSubstantially freexe2x80x9d means that a maximum of 10 percent, preferably a maximum of 5 percent, of the total epoxy groups of the powder coating compositions according to the invention is provided by TGIC or glycidyl esters. Finally, most preferred are powder coating compositions according to the invention that are completely free of glycidyl compounds, especially free of TGIC and glycidyl esters.
Suitable binders for the powder coating compositions according to the invention include, for example, free-carboxyl-group-containing polyesters having an acid number of from 10 to 160 mg, preferably from 10 to 70 mg, especially from 20 to 40 mg, of KOH per kilogram of polyester.
The polyesters are furthermore advantageously solid at room temperature (from 15 to 35xc2x0 C.) and have, for example, a molecular weight (number average Mn) of from 1000 to 10 000. The ratio of Mw (weight average of the molecular weight) to Mn of those polyesters is generally from 2 to 10. There are especially suitable, for example, free-carboxyl-group-containing polyesters having a molecular weight (weight average Mw from GPC measurement using polystyrene calibration) of from 4000 to 15000, especially from 6500 to 11000, and a glass transition temperature (Tg) of from 35 to 120xc2x0 C., preferably from 50 to 90xc2x0 C.
Polyesters such as those mentioned are described, for example, in U.S. Pat. No. 3 397 254 and EP-A-0 600 546. Polyesters suitable for the present invention are condensation products of difunctional, trifunctional and/or polyfunctional alcohols (polyols) with dicarboxylic acids and, optionally, trifunctional and/or polyfunctional carboxylic acids, or with corresponding carboxylic acid anhydrides. The polyols used include, for example, ethylene glycol, diethylene glycol, the propylene glycols, butylene glycol, 1,3-butanediol, 1,4-butanediol, neopentanediol, isopentyl glycol, 1,6-hexanediol, glycerol, hexanetriol, trimethylolethane, trimethylolpropane, erythritol, pentaerythritol, cyclohexanediol and 1,4-dimethylolcyclohexane. Suitable dicarboxylic acids include, for example, isophthalic acid, terephthalic acid, phthalic acid, methyl-substituted derivatives of the said acids, tetrahydrophthalic acid, methyltetrahydrophthalic acids, for example 4-methyltetrahydrophthalic acid, cyclohexane-dicarboxylic acids, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, fumaric acid, maleic acid and 4,4xe2x80x2-diphenyl-dicarboxylic acid etc.. Suitable tricarboxylic acids include, for example, aliphatic tricarboxylic acids, such as 1,2,3-propanetricarboxylic acid, aromatic tricarboxylic acids, such as trimesic acid, trimellitic acid and hemimellitic acid, and cycloaliphatic tricarboxylic acids, such as 6-methylcyclohex-4-ene-1,2,3-tricarboxylic acid. Suitable tetracarboxylic acids include, for example, pyromellitic acid and benzophenone-3,3xe2x80x2,4,4xe2x80x2-tetracarboxylic acid. Commercially available polyesters especially are very commonly based on neopentyl glycol and/or trimethylolpropane as the main alcoholic monomer constituent(s) and on adipic acid and/or terephthalic acid and/or isophthalic acid and/or trimellitic acid as the main acidic monomer component(s).
Also suitable as binders are carboxyl-group-containing poly(meth)acrylates, which can be prepared in known manner by the copolymerisation of acrylic and/or methacrylic monomers, for example, C1-C12alkyl(meth)acrylates, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, 2-ethylhexyl, decyl and dodecyl(meth)acrylates, C1-C4alkyl(meth)acrylates being preferred, or (meth)acrylamide with acrylic acid and/or methacrylic acid and, where appropriate, other ethylenically unsaturated comonomers, such as vinyl aromatic compounds, e.g. styrene, xcex1-methylstyrene, vinyltoluene or also xcex2-halogenated styrenes, in addition. The copolymerisation can be carried out in known manner. For example, the monomers can be dissolved in suitable organic solvents and thermally reacted in the presence of a suitable initiator that is soluble in the solvent, such as dicumyl peroxide, and in the presence of a suitable chain-transfer reagent, such as thioglycolic acid (solution polymerisation), or the monomer mixture can be suspended in water together with a solution of the initiator in an organic solvent and polymerised, or the monomer mixture can also be emulsified in water with the aid of surfactants, e.g. sodium lauryl sulfate, and reacted in the presence of a water-soluble polymerisation initiator, such as K2S2O8 (emulsion polymerisation). The prepared poly(meth)acrylic resin is in each case then isolated in solid form from the solvent or water. The reaction can also be carried out without using solvents or water, for example according to JP-A-Sho 53-140 395. Suitable poly(meth)acrylic resins are solid at temperatures in the region of room temperature (from 15 to 25xc2x0 C.). They generally have a molecular weight of from 1000 to 50000 (weight average Mw), preferably from 5000 to 20000.
The Tg value (glass transition temperature) of the poly(meth)acrylates, determined by DSC (heating rate 10xc2x0 C./minute), is preferably from 40 to 75xc2x0 C. The acid number of the resins, quoted in mg equivalent KOH per g of (meth)acrylate resin, is preferably from 20 to 160, especially from 20 to 80.
In certain cases it may also be advantageous to use, as binders, a mixture of free-carboxyl-group-containing polyesters and free-carboxyl-group-containing poly(meth)acrylates.
The powder coating compositions according to the invention comprise epoxy compounds and binders preferably in such an amount that the ratio of epoxy groups to carboxyl groups of the binder is from 2:1 to 0.5:1, preferably from 1.3:1 to 0.7:1. The compositions according to the invention may especially have a slight molar excess of epoxy groups. The molar ratio of epoxy groups to carboxyl groups in the compositions is thus preferably from 1.3:1 to 1:1, e.g. approximately from 1.2:1 to 1.1:1.
Preferably, the powder coating compositions according to the invention also comprise a catalyst for the reaction of epoxy groups with carboxyl groups. Such a catalyst is commonly an organic amine or a derivative of an amine, especially a tertiary amine or a nitrogen-containing heterocyclic compound. Preferred catalysts for the reaction of epoxy groups with carboxyl groups are phenylimidazole, N-benzyldimethylamine and 1,8-diazabicyclo[5.4.0]-7-undecene, optionally on a silicate support or triphenylphosphine, alkyltriphenylphosphonium halide, Actirone(copyright) NXJ-60 (2-propylimidazole), Actiron(copyright) NXJ-60 P (60% by weight of 2-propylimidazole on 40 % by weight of solid support), Beschleuniger(copyright) DT 3126 (alkylammonium salt in polyester). The catalyst or a catalyst mixture is preferably added in such an amount that the gel time of the mixture at 180xc2x0 C. (determined according to DIN 55990) is approximately from 70 to 400 seconds, preferably from 90 to 300 seconds. Generally, approximately from 0.1 to 10 percent by weight, especially from 0.5 to 5 percent by weight, of catalyst will be required for that purpose. Of course some commercially available polyesters that can be used as binders for the powder coating compositions according to the invention will already contain a certain amount of one of the above-mentioned catalysts or of a comparable catalyst, and that amount should be taken into account in the above percentage by weight figure for the catalyst; the mentioned preferred gel times can be used to provide an indication of how much catalyst still needs to be added.
The powder coating compositions according to the invention may also comprise further additives customary in the surface-coating industry, for example light stabilizers, dyes, pigments, for example titanium dioxide pigment, degassing agents, for example benzoin, and/or flow agents. Suitable flow agents include, for example, polyvinyl acetals, such as polyvinyl butyral, polyethylene glycol, polyvinylpyrrolidone, glycerol and acrylic mixed polymers, such as, for example, those available under the names Modaflow(copyright) and Acrylron(copyright).
Powder coating compositions according to the invention can be prepared simply by mixing the constituents together, for example in a ball mill. Another, more preferred possibility comprises melting together, blending and homogenising the constituents, preferably using an extrusion machine, such as a Buss co-kneader, and cooling and comminuting the resulting mass. In that procedure, the fact that either immediately after extrusion, or at least after they have been left to stand for a few hours, for example from 24 to 48 hours, the powder coating compositions according to the invention become so hard and brittle that they can readily be ground, has proved especially advantageous. The powder coating composition mixtures preferably have a particle size in the range from 0.015 to 500 xcexcm, especially from 10 to 75 xcexcm. In some cases it may also be advantageous first of all to prepare a masterbatch from portions of the binder, the epoxy resins and, optionally, further components, the masterbatch then being mixed and homogenised in a second step with the remainder of the binder and the remaining constituents to yield the finished powder coating composition.
After application to the article to be coated, the powder coating compositions are cured at a temperature of at least approximately 100xc2x0 C., for example from 150 to 250xc2x0 C. Curing generally takes approximately from 10 to 60 minutes. All materials that are stable at the temperatures required for the curing, especially ceramics and metals, are suitable for coating. The substrate may already have one or more base surface-coatings that are compatible with the powder coating composition.
The powder coating compositions exhibit good flow behaviour combined with good mechanical properties, good weather resistance and good resistance to chemicals.